1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same. The present invention is suitable for image pickup apparatuses that use an image sensor, such as a digital still camera, a video camera, a monitoring camera, or a broadcasting camera, and image pickup apparatuses, such as a camera that uses silver-halide film.
2. Description of the Related Art
Image pickup apparatuses, such as a digital still camera and a video camera, that use a solid-state image sensor have recently increased in performance and decreased in size. To that end, zoom lenses for use in such apparatuses are required to have a short overall lens length and high magnification ratios. As a zoom lens that meets such requirements, a positive-lead type lens is known in which a lens unit having positive refractive power is disposed closest to an object side.
Japanese Patent Application Laid-Open No. 2006-171655 discloses a four-unit zoom lens which consists of lens units having positive, negative, positive, and positive refractive power in this order from an object side to an image side, and a five-unit zoom lens which consists of lens units having positive, negative, positive, positive, and positive refractive power in this order from the object side to the image side. Japanese Patent Application Laid-Open No. 2003-255228 discloses a five-unit zoom lens which consists of lens units having positive, negative, positive, negative, and positive refractive power in this order from the object side to the image side. In a zoom lens disclosed in Japanese Patent Application Laid-Open No. 2006-171655, the first to fourth lens units are moved for zooming. In a zoom lens disclosed in Japanese Patent Application Laid-Open No. 2003-255228, the first to fifths lens units are moved for zooming.
In general, to obtain a compact, high-magnification image-pickup optical system, the number of lenses that constitute individual lens units is reduced while the refractive power of the individual lens units constituting the image-pickup optical system is increased. However, increasing the refractive power of the lens units increases the refractive power of the surfaces of the lenses, resulting in an increase in the thicknesses of the lenses to ensure sufficient edge thicknesses of the lenses. This increases the effective diameter of a front lens, thus making it difficult to obtain a compact image-pickup optical system. Furthermore, this causes various aberrations, such as chromatic aberration at a telephoto end, which are difficult to correct.